The invention relates to a receiver for angle-modulated data signals of the type a sin ((.omega.)t+.phi.(t)), wherein .omega. represents the carrier frequency and .phi.(t) the data-dependent angle modulation of the carrier, comprising a demodulation channel having an output for a signal which is a function of the modulation signal .phi.(t), the demodulation channel comprising a frequency transposition stage for providing an output signal proportional to sin ((.DELTA..omega.)t+.phi.(t)) comprising a local carrier oscillator having a frequency which may deviate by an amount (.DELTA..omega.) from the carrier frequency of the angle-modulated signals applied to the demodulation channel.
Such a receiver is generally known, for which reference is made to IEEE Transactions on Communications, Volume Com-20, No. 3, June 1972, pages 429-35 (de Buda) and Vol. Com. 26, No. 5, May 1978, pages 534-42 (de Jager et al.).
In receivers of this type it is customary to adjust the frequency of the local oscillator so that the frequency difference .DELTA..omega. between the local oscillator and the received carrier disappears. Such a control circuit is described in, for example, the above-mentioned article by de Buda. A further example is given in Proceedings of the IRE, Vol. 44, No. 12, 1956, pages 1713-8 (Costas). It appears that in the case of a large initial frequency difference .DELTA..omega., these control circuits have a relatively long adjusting period, which furthermore depends on the signal-to-noise ratio.
For a receiver having two quadrature channels, it has been proposed to sample a reference signal with the symbol clock frequency and with the symbol clock phase at instants which coincide with the zero passages of the demodulated X- and/or Y-signal, to provide a phase control signal for the local oscillator. This method is particularly suitable for angle modulation systems in which the phase of the carrier in a symbol interval changes a predetermined defined amount (for example, 0, .pi./4, .pi./2) as, for example, described in the above-mentioned article by de Jager et al. In accordance with an alternative proposition the mutual distances between the zero passages of the X-signal and/or the Y-signal is determined and examined for deviations with respect to the symbol period or multiples thereof, to provide a phase control signal. This does not require knowledge of the exact symbol clock phase. Due to the dependence on the zero passages of the demodulated signal, these control methods depend to a very great extent on the noise and have a limited pull-in range at low signal-to-noise ratios.